Application Notes: continued
A rule of thumb useful in determining if a protection diode The thermal characteristics of an IC depend on the follow-
is required is to solve for current
ing four factors: junction temperature, ambient tempera-
ture, die power dissipation, and the thermal resistance
from the die junction to ambient air. The maximum junc-
tion temperature can be determined by:
I= C ´ V , where
T
I
is the current flow out of the load capacitance
when VCONTROL is shorted,
TJ(max) = TA(max) + PD(max) ´ RQJA
C
V
T
is the value of load capacitance
The maximum ambient temperature and the power dissi-
pation are determined by the design while the maximum
junction temperature and the thermal resistance depend on
the manufacturer and the package type. The maximum
power dissipation for a regulator is:
is the output voltage, and
is the time duration required for VCONTROL
to transition from high to being shorted.
If the calculated current is greater than or equal to the typi-
cal short circuit current value provided in the specifica-
tions, serious thought should be given to the use of a pro-
tection diode.
PD(max) = (VIN(max) -VOUT(min))IOUT(max) + VIN(max) ´ IIN(max)
A heat sink effectively increases the surface area of the
package to improve the flow of heat away from the IC and
into the surrounding air. Each material in the heat flow
path between the IC and the outside environment has a
thermal resistance which is measured in degrees per watt.
Like series electrical resistances, these thermal resistances
are summed to determine the total thermal resistance
Current Limit
The internal current limit circuit limits the output current
under excessive load conditions.
between the die junction and the surrounding air, RQJA
.
Short Circuit Protection
This total thermal resistance is comprised of three compo-
nents. These resistive terms are measured from junction to
case (RQJC), case to heat sink (RQCS), and heat sink to ambi-
ent air (RQSA). The equation is:
The device includes short circuit protection circuitry that
clamps the output current at approximately two amperes
less than its current limit value. This provides for a current
foldback function, which reduces power dissipation under
a direct shorted load.
R
QJA = RQJC + RQCS + RQSA
The value for RQJC is 1.4ûC/watt for the CS5257A-1 in both
the TO-220 and D2PAK packages. For a high current regu-
lator such as the CS5257A-1 the majority of heat is generat-
ed in the power transistor section. The value for RQSA
depends on the heat sink type, while the RQCS depends on
factors such as package type, heat sink interface (is an
insulator and thermal grease used?), and the contact area
between the heat sink and the package. Once these calcula-
tions are complete, the maximum permissible value of
Thermal Shutdown
The thermal shutdown circuitry is guaranteed by design to
activate above a die junction temperature of approximately
150¡C and to shut down the regulator output. This circuit-
ry has 25¡C of typical hysteresis, thereby allowing the reg-
ulator to recover from a thermal fault automatically.
R
QJA can be calculated and the proper heat sink selected.
Calculating Power Dissipation
and Heat Sink Requirements
For further discussion on heat sink selection, see our
Cherry application note ÒThermal Management for Linear
Regulators.Ó
High power regulators such as the CS5257A-1 usually
operate at high junction temperatures. Therefore, it is
important to calculate the power dissipation and junction
temperatures accurately to ensure that an adequate heat
sink is used. Since the package tab is connected to Vout on
the CS5257A-1, electrical isolation may be required for
some applications. Also, as with all high power packages,
thermal compound in necessary to ensure proper heat
flow. For added safety, this high current LDO includes an
internal thermal shutdown circuit
8
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
